This application is the national phase under 35 U.S.C. xc2xa7371 of PCT International Application No. PCT/IB99/02093 which has an International filing date of Dec. 22, 1999, which designated the United States of America and was published in English.
The present invention relates to a biocompatible implant consisting essentially of a metal such as titanium, zirconium, hafnium and tantalum, or an alloy thereof, the surface of which has been modified in order to increase ,the biocompatibility. The invention also relates to a method modification of surfaces to.
Titanium, zirconium, hafnium and tantalum and their alloys have a superb corrosion resistance in body fluids and are well accepted by the human body. Titanium and its alloys are therefore much used for implants. In many applications it is of utmost importance that the metal form a strong and lasting connection with the surrounding tissues and that this connection does not impair healing. This is not easy to achieve. Implant materials not giving satisfactory healing usually lead to loss of contact between the implant and tissue, often followed by complications leading to implant failure. This has given the patients severe pain and required costly medical treatment often including complicated and expensive surgery.
To deal with these problems geometric modifications of implants have been applied. Increasing the surface roughness expands the area of tissue contact. Different methods including plasma spraying, sand blasting or creation of holes or grooves to establish an inter-locking effect in the bone have achieved this. Electron beam machining has been used to make surfaces that hardly can be produced with conventional machining. These latter methods can be optimised to also give additional geometrical advantages. Another method commonly used is to apply a layer of hydroxyapatite coating onto the titanium implant surface. This mineral is present in hard tissue of all mammals. All these techniques are manufacturing- and user- sensitive and it is problematic to carry out coating in a way that gives sufficient bonding between the mineral and the metal. Another serious disadvantage with these techniques is destruction of the mineral coating during applications where stress is applied to the implant. This seriously hampers applications of metal implants.
In contact with oxygen titanium, zirconium, hafnium and tantalum and their alloys are instantaneously covered with a thin layer of oxide. Various techniques exist to increase the thickness of the oxide layer. Significant improvements have not been obtained so far, concerning the biocompatibility of the implant material. The oxide layer may be further treated. For example EP-A-0 264 354 describes a process for forming a coating of a calcium phosphate compound on the surface of the titanium oxide layer. In the process to obtain the desired oxide layer it is possible to use either acid treatment or formation of an intermediate metal hydride, which is then heated in order to obtain the desired oxide as a substrate for the calcium phosphate coating.
Another method for treating the surface of endosseous implants is to use the process described in EP-A-0 212 929, according to which a ceramic material is thermally sprayed onto the metal surface after its been roughened with an appropriate technique. The roughening of the metal surface may be obtained by e.g. thermally spraying titanium hydride onto it, but, as for EP-A-0 264 354, the titanium hydride coated implant is only an intermediate product in the process of obtaining the desired end product, in this case the ceramic coated implant.
The object of the present invention is to provide an implant with improved biocompatibility compared to known implants. This is obtained by modifying the surface of the implant. The modified surface further promotes contact between tissue ant implant. In the research work leading to the present invention it was surprisingly observed that implants coated with titanium hydride led to a better adherence between the metal and bone, compared to other titanium implants. The fact that titanium hydride coated implants could be used directly is very surprising; up to the present invention it has been considered necessary to coat hydrided surfaces to achieve satisfactory biocompatibility. In the work leading to the present invention it was demonstrated in animal models that tissues in contact with the titanium hydrided titanium surface was healthy and showed no foreign body reactions as examined by microscopy.
The present invention thus relates to biocompatible metallic implants, characterized in that the surfaces of the implants have been modified so that they comprise a metal hydride layer.
The invention also relates to a method for the production of a biocompatible implant, wherein a core of metal or an alloy thereof is coated with a surface layer of hydride.
The characterizing features of the invention will be evident from the following description and the appended claims.
As stated above, the invention relates to a biocompatible implant consisting essentially of metal or an alloy thereof, characterized in that the surface of the implant is modified, preferably so that it comprises an outer layer, preferably essentially consisting of a metal hydride. The expression xe2x80x9cbiocompatible implantxe2x80x9d used herein relates to implants suitable for introduction into the body of a mammal, and especially of a human. The implants according to the invention or implants produced with the method according to the invention are intended for introduction into all living hard and soft tissues, including bone, cartilage and teeth, and all body cavities including joints and inner ear.
The hydride layer in the implant according to the invention may be any metal hydride or a mixture of several different metal hydrides.
In the case of an implant of titanium or an alloy thereof the major part of the modified outer layer, i.e. more than 50%, is preferably constituted by TiH1.924 or TiH2. This titanium hydride layer may also comprise small amounts of other elements and hydrides thereof.
The invention also relates to a method suitable for the production of the above described biological implant. This method results in an implant surface, which comprises a layer of hydride. This may be performed either by coating with a layer of hydride, or by converting the surface into hydride. It is possible to use a commercially available implant and convert its surface to comprise a hydrided layer. It is also possible to produce the implant according to the invention, by first producing a suitably shaped core of titanium or an alloy thereof, and then accomplish the titanium hydride layer.
The method according to the invention is preferably performed by treating the starting implant or core by electrolysis. The starting implant is then placed in an electrolytic bath. During the electrolysis, the starting implant will constitute the cathode.
The electrolytic bath is preferably an aqueous solution of NaCl with acidic pH-value. The pH is preferably adjusted to the appropriate value by addition of HCl, H2SO4, HNO3, HClO4 , or an organic acid or a mixture of two or more of these acids.
The temperature of the electrolytic bath should also be adjusted. It is possible to perform the method according to the invention at ambient temperature, i.e. at approximately 20xc2x0 C., however, at this temperature the reaction rate will be very slow. In order to increase the reaction rate, the temperature should be raised, preferably to at least 40xc2x0 C., and most preferably to at least 80xc2x0 C.
The most preferred electrolytic solution for use in the method according to the invention is an aqueous solution comprising from 0,01 M to 1 M of a saturated salt solution and from 10xe2x88x925 to 10 M of at least on of the above mentioned acids.
The current used to perform the electrolysis is 0.001-1000 mA/cm2.
In order to further improve the biocompatability of the implant it is to be implanted into, it is advantageous to increase the surface roughness of the hydride layer. This can for example be done by blasting, e.g. grit blasting, before hydriding the implant.
The invention will now be further explained in the following examples. These examples are only intended to illustrate the invention and should in no way be considered to limit the scope of the invention.